It is well known that addition of titanium to aluminum alloys causes grain refinement of the resulting castings through nucleation of alpha aluminum by the primary Al3Ti phase which forms via the peritectic reaction. Additions of boron were shown to remarkably improve grain refinement of aluminum by titanium at hypoperitectic concentrations. A. Cibula, J. Inst. Met., 76 (1949-1950) 321-360. As a result, Al—Ti—B master alloys emerged as potential grain refiners for aluminum alloys. At present, there is a variety of commercial grain refiners of this type. Examples of these alloys are disclosed in U.S. Pat. Nos. 3,857,705, 4,298,408, 4,612,073 and 4,873,054. Various methods for the production of Al—Ti—B grain refiner alloys have been described in U.S. Pat. Nos. 6,228,185, 5,415,708, 5,484,493, 3,961,995, 3,785,807, 5,104,616, GB-A-2,257,985, GB-A-2,259,308 and GB-A-2,259,309 as well as in numerous papers. D. G. McCartney, Int. Mater. Rev., 34 (1989) 247. B. S. Murty et al., J. Mater. Process. Tecnol., 89-90 (1999) 152-158. B. S. Murty et al., Int. Mater. Rev., 47 (2002) 3-29. M. S. Lee and B. S. Terry, Mater Sci. Technol., 7 (1991) 608-612; M. J. Jackson and I. D. Graham, J. Mater. Sci Lett., 13 (1994) 754-756; M. S. Lee, B. S. Terry and P. Grieveson, Metall. Trans. B., 24B (1993) 955-961; Q. Zhuxian et al., Aluminium, 64 (1988) 1254-1257; I. G. Davies et al., Metall. Trans., 1 (1970) 275-280; I. Maxwell and A. Hellawell, Acta Metall., 23 (1975) 895-899, K. A. Q. O'Reilly et al., Scr. Metall. Mater., 28 (1993) 173-177; T. S. Krishnan et al., J. Alloy. Compd., 269 (1998) 138-140; M. G. Chu, Mater. Sci. Eng., A179-180 (1994) 669-675. C. S. Sivaramakrishnan and R. Kumar, Light Metal Age, 10 (1987) 30-34. C. D. Mayes and D. G. McCartney, Mater. Sci. Tech., 9 (1993) 97-103. M. M. Guzowski, et al., Metall. Trans., 18A (1987) 603-619.
The present invention describes a process to synthesize Al—Ti—B alloys with the insoluble AlB2 and the soluble Al3Ti particles to maximize the grain refining efficiency with aluminium foundry alloys. It relies on a solid-state reaction between aluminium and K2TiF6 to generate Al3Ti particles in a mixture which already has preformed AlB2 particles. The more stable of the two potential borides, TiB2, is inevitably favored when KBF4 and K2TiF6 salts are added to molten aluminium. Even when the halide salts are added sequentially so as to form first AlB2, one would expect AlB2 to transform to TiB2 as soon as K2TiF6 is added in the melt, according to, 3K2TiF6+3AlB2+Al®3TiB2+3KAlF4+K3AlF6, since TiB2 is more stable than AlB2. The process of the present invention not only avoids the AlB2 to TiB2 transformation, but also offers exceptional microstructural features. Al3Ti particles generated by a solid state reaction between K2TiF6 and aluminium are much smaller than those available in Al—Ti/Al—Ti—B master alloys prepared with prior art yielding a superior grain refining performance.
The present invention offers a process for the production of Al—Ti—B grain refiner master alloys, containing from 1 to 10% titanium, 0.2 to 3% boron and the balance essentially aluminum, wherein the resultant alloy contains Al3Ti particles having a diameter of less than 20 microns and a fine dispersion of AlB2 particles. The process of the present invention also relies on the reaction of halide salts with aluminum to produce Al—Ti—B grain refiner master alloy, yet is different from the prior art as it is a powder metallurgy process and takes place in the solid state. The present invention yields smaller Al3Ti particles which ensure a fast grain refining response and AlB2, instead of TiB2 particles. The Al—Ti—B grain refiner alloys produced according to the present invention provided consistent and better overall grain refining performance with respect to those prepared with the prior art.
A sound process to produce a Al—Ti—B master alloys which ensure an adequate grain refining performance for aluminium foundry alloys is claimed to comprise the following steps: Mixing Al—B alloy powder and K2TiF6 salt thoroughly to obtain a blended mixture; heating the mixed powder blend thus obtained under flowing argon to slightly below the melting point of aluminium, i.e. 650 degrees Celcius, and holding it at this temperature sufficiently long, i.e. for ½ hours. Inoculation with the said alloys has produced a fine equiaxed grain structure across the entire section of the test sample which was more or less retained for 15 minutes after inoculation. Besides, the dendritic as-cast structure is improved into a more homogeneous one, dominated by equiaxed a —Al rosettes.